STORY ARCHIVE

GPS

TRANSCRIPT

Paul WillisAs with so many of my modern gadgets, I've only got a vague idea how GPS works.

GPSGo straight ahead. You have reached your destination

NARRATIONToday, I'm on a journey of understanding.

Paul WillisThorsten.

Dr Thorsten KostulskiHello Paul.

Paul WillisNice to meet you.

NARRATIONDr Thorsten Kostulski is a telecommunications engineer he'll know how it works.

Paul WillisNow how does this thing actually work? How does it know where we are?

Dr Thorsten KostulskiWell it receives signals from GPS satellites which are organised in six orbital planes in an inclination of 55 degrees around the earth.

NARRATIONLet's see if we can break this down. Here's our car with a GPS receiver and here's a satellite. The satellite sends a radio signal at the speed of light, containing a code that identifies the satellite, along with its precise orbital location and the time the message was sent. Using this information, the receiver can calculate the distance to the satellite. But this gives a circle of possible locations on earth. A second satellite reduces this to two possible locations, where the circles intersect.A third satellite fixes one unique location. And this is how your GPS unit knows where you are, wherever you travel on the planet.

Dr Thorsten KostulskiBut there's a little problem.

Paul WillisThere's a problem?

Dr Thorsten KostulskiThe problem is if your clock is off by only one microsecond this multiplied by the speed of light you could be as far as 300 metres away from where you think you are.

NARRATIONWhat we need is a very, very accurate clock.

Dr Thorsten KostulskiThis is an atomic clock which is the most accurate way of measuring time. There is one of these aboard each of the GPS satellites and they cost around $100,000 each.

Paul WillisWell, at that price you don't get one in every mobile phone.

NARRATIONA fourth satellite reduces the error of the inferior clock in our cheap GPS device giving a more accurate location. That's why there must always be at least four GPS satellites in view. But that's not always possible.